(1) Field of the Invention
The present invention is directed generally towards a system for transmitting acoustic fields underwater and more specifically towards a system for transmitting acoustic fields underwater from a cable towed by a ship or airborne device.
(2) Description of the Prior Art
A ship such as a military ship can tow an array of sound receiving hydrophones arranged in a passive towed array. The passive towed array, in conjunction with sound receiving and signal processing electronics, can detect sounds in the water that may indicate, for example, the presence of an enemy submarine.
In other arrangements, the ship can tow both the passive towed array of sound receiving hydrophones and also a towed acoustic projector, which together form a bi-static active sonar system. With this arrangement, the towed acoustic projector emits sound pulses. Each sound pulse travels through the water, striking objects or targets in the water, which in turn produces echoes. The echoes are received by the towed array of receiving hydrophones. Therefore, an echo indicates the presence of an underwater object, and the direction from which the echo came indicates the direction of the underwater object.
In conventional bi-static active sonar systems, the towed acoustic projector is often deployed and towed separately from the towed array of sound receiving hydrophones. A conventional towed acoustic projector typically includes a sound source mounted within a large rigid tow body. The conventional towed acoustic projector is large and heavy. For example, the size can be on the order of forty-eight cubic feet and the weight can be on the order of 4000 pounds. The towed acoustic projector of this type is typically used to detect objects in deep water and at long ranges. Therefore, the towed acoustic projector is capable of generating sound having a high pressure level, (i.e., 220 decibels relative to one microPascal at a distance of one meter) in order to enable the system to receive echoes from and to detect objects in the deep water at long ranges.
The towed acoustic projector is deployed and recovered over the gunwale of the ship with winch and boom equipment. The large towed acoustic projector, which requires deployment over the gunwale of the ship, limits covert deployment and recovery. The deployment and recovery of the towed acoustic projector over the gunwale of the ship may also be unwieldy and dangerous during deployment and recovery in high seas.
Conventional bi-static active toward array systems are configured such that the acoustic projector is handled with equipment separate from equipment that handles the towed array of receiving hydrophones. Large handling equipment is needed to deploy and recover the towed acoustic projector. The large handling equipment requires a considerable amount of deck space on a ship. The large handling equipment is undesirable not only because of the deck space that it requires, but also because it presents a radar target having a larger radar cross section to enemy radar systems.
U.S. Pat. No. 6,683,819 discloses a sonar array system including a receiving array of hydrophones and a transmit array of acoustic projectors disposed on a common axis via a cable. The cable is extended and retrieved by use of a winch. The transmit array has one or more acoustic projectors capable of generating sound and the receive array has one or more hydrophones capable of receiving sound. The one or more acoustic projectors are disposed on a transmit array axis, and the one or more hydrophones are disposed on a receive array axis. The receive array axis and the transmit array axis have a common axis, therefore forming the contiguous line arrays, one for receiving and one for transmitting. The sonar array can be towed to form a horizontal line array having both acoustic projectors and hydrophones. The acoustic projectors and hydrophones can be deployed and recovered using a single winch.
The U.S. Navy used variable depth sonar (VDS) systems towed behind ships in the 1990s. These VDS systems are large, heavy, complex systems that required a large handling system with a dedicated room and a stern door. The heavy tow body led to a steep critical angle for the tow cable, requiring fairing and a specialized winch with multiple drums. The winch and foundation rating was required to exceed a typical cable breaking strength of 100,000 pound.